Host‐pathogen dialogues in different cell death modes during Mycobacterium tuberculosis infection

Tuberculosis (TB) is a fatal infectious disease that continues to pose a serious public health threat. The emergence of drug‐resistant TB has further worsened the burden of the disease. The interaction between the host and the pathogen involves multiple modes of cell death, which play a role in determining immune outcomes. Several studies have established a strong correlation between cell death and TB progression. This review explores the molecular mechanisms of various cell death modes in Mycobacterium tuberculosis (Mtb) infection and how Mtb's virulence effectors regulate these pathways, including apoptosis, autophagy, pyroptosis, ferroptosis, and necrosis. Furthermore, therapeutic strategies targeting cell death pathways have shown promising results in TB treatment. Importantly, this review highlights the significant potential of mitochondria in mediating communication between different cell death modes and influencing the final outcomes. Overall, this work provides a comprehensive summary of the role of cell death in host immune responses and immune evasion by Mtb. It also offers valuable insights into the pathogenesis of TB and immune evasion strategies employed by Mtb and contributes to the development of more effective anti‐TB therapies.

phagocytosed and eliminated by alveolar macrophages.Both innate and adaptive immunity are involved in regulating Mtb infection, however, Mtb is a highly adaptable intracellular bacterium that has coevolved with humans over thousands of years, successfully evading the host's immune response without causing typical disease.Additionally, it triggers a strong inflammatory response, resulting in widespread pathology. 3Understanding the mechanisms by which the pathogen evades the immune system is crucial for controlling TB, and it is reported that modulation of host cell death is one of the most important ways. 4,5ell death is a tightly regulated process in embryonic development, crucial for maintaining intra-tissue homeostasis by eliminating damaged tissues and potentially harmful cells. 6Cell death underlies host-pathogen interactions, with the discovery of multiple cell death types, an increasing number of studies have begun to focus on the role of cell death in host immunity against tuberculosis and pathogen immune escape. 7In 2018, the Nomenclature Committee on Cell Death redefined and refined the cell death classification based on morphological, biochemical, and functional perspectives. 6arious types of cell death are involved in Mtb infection, leading to distinct infection outcomes. 8,9Most studies have concluded that apoptosis, pyroptosis, and autophagy contribute to the inhibition of pathogen survival, while necrosis and ferroptosis are the main modes of promoting Mtb replication and dissemination. 8However, apoptosis has been shown in recent studies to promote the survival and release of Mtb, 10 suggesting that the immune outcomes of host cell death modes in a given situation may be multifaceted.Additionally, to address the challenges posed by drug-resistant TB, host-directed therapies (HDTs) targeting host cell death have become an increasing focus of research. 11HDTs differ from conventional treatments and are less likely to lead to resistance.When combined with conventional anti-TB drugs, they can shorten treatment cycles, reduce relapses and minimize toxicity without compromising antibiotic efficacy. 12A deeper understanding of the molecular mechanisms of host cell death could help develop more effective anti-tuberculosis drugs.
Although our understanding of the impact of Mtb factors on host cell death has significantly expanded, 13,14 the molecular mechanisms that regulate the different modes of death and the impact on the outcome of infection remain unclear.This review highlights recent advances in apoptosis, autophagy, pyroptosis, ferroptosis, and necrosis during Mtb infection and discusses how Mtb regulate different death modes for immune escape.A deeper understanding of the molecular dialogs between the host and Mtb will be crucial for treating TB and other chronic infectious diseases.

| APOPTOSIS
Apoptosis is a highly regulated mode of programmed cell death (PCD) associated with various diseases. 15There are two pathways of apoptosis, namely the endogenous (mitochondrial) pathway and the exogenous (death receptor) pathway. 6Upstream stimulatory signaling facilitates the activation of promoter and effector associations, leading to a range of typical apoptotic phenotypes. 16These phenotypes include cell shrinkage, DNA fragmentation, chromatin condensation, actomyosin-driven membrane blebbing, and production of apoptotic vesicles. 17Apoptosis triggers phagocytosis and recognition by neighboring immune cells without causing damage to surrounding tissue cells. 18In general, macrophage apoptosis is a protective strategy for host elimination of pathogens; however, the outcome of apoptosis becomes more complex when Mtb infects macrophages.Meanwhile, various host and pathogen effectors are in response to Mtb infection.

| Host-induced apoptosis in response to Mtb infection
Apoptosis is a crucial mechanism employed by the host to prevent intracellular replication of Mtb. 5,19,20The mechanisms by which Mtb induces apoptosis in the host have been extensively studied.
Upon Mtb infection, a variety of host proteins and molecules are involved in inducing the endogenous pathway and the exogenous pathway.2][23] On the other hand, TNF-α, 24 toll-like receptor (TLR), 25 Fas/APO-1 CD 95 receptor, 26 IFN-γ, 27 CCL20, 28 and caspase 29 are involved in exogenous apoptosis.These host apoptotic proteins respond to pathogen infection and kill Mtb by inducing apoptosis.
In addition to classical apoptotic proteins, dualspecificity phosphatases are members of the protein tyrosine phosphatase superfamily, which can participate in BCG-induced THP-1 MAPKs/NF-κ B signaling and mediate apoptosis. 30Another important approach for pathogens and hosts to remodel transcriptomes is alternative splicing.Recent studies have shown that UBE2B, a ubiquitin-conjugating enzyme, and host UBE2B-exon7skipping exon (SE) predict tuberculosis and are closely associated with apoptosis onset. 31

| Mtb effectors induce host apoptosis
The roles of cell wall components, secreted proteins, and the proline-glutamate/proline-proline-glutamate (PE/ PPE) family proteins of Mtb have been extensively demonstrated in inducing host cell apoptosis (Table 1).In terms of cell wall components, Lipomannan and lipoarabinomannan (LAM) have been found to induce apoptosis and IL-12 secretion in THP-1. 32The 19-kDa M. tuberculosis glycolipoprotein (p19) activates THP-1 apoptosis through Toll-like receptor 2 ,TLR2 signaling, inhibiting bacterial survival and depending on caspase 8/9 activation. 33Additionally, the cell wall proteins Rv0901 34 and 38-kDa lipoprotein Rv0934 35 both induce apoptosis.In terms of secreted protein, Early Secreting Antigen Target-6kDa, ESAT-6 has been found to promote caspase gene expression and the formation of membrane pores, leading to exogenous apoptosis. 36Further studies have shown that ESAT-6 increases miR-155 expression through the TLR2/NF-kB/SOCS1 signaling axis, promoting apoptosis. 62In terms of PE/PPE family proteins, PE-PGRS33 induces the release of TNF-a and activates ASK1 through the TLR2 pathway, subsequently activating P38 and JNK and promoting host apoptosis. 37However, PE9 (Rv1088)-PE10 (Rv1089) induces macrophage apoptosis through TLR4. 38Additionally, the hemolytic phospholipase lip gene (Rv0183) increases the expression of IL-6, NF-κB, TLR-2, TLR-6, TNF-α, and Myd88 and promotes apoptosis in RAW264.7. 39t is interesting that host apoptosis not only inhibits Mtb survival but also enhances bacterial survival and dissemination.For example, the CDP-diglyceride hydrolase of Mycobacterium (CdhM), a protein located in the endoplasmic reticulum, facilitates the spread of Mtb through endoplasmic reticulum stress-induced host apoptosis. 40Another group of proteins are PE/PPE protein family members; most of them can target and interfere with mitochondria to regulate the outcome of host apoptosis.For example, the PE6 protein (Rv0335c) activates the TLR4 receptor and induces inflammation through the NF-κB signaling pathway, while targeting mitochondria to induce caspase-mediated apoptosis and enhance Mtb survival. 41

| Mtb effectors inhibit host apoptosis
The survival and persistence of Mtb rely on its ability to manipulate various host defense pathways, including the ability to actively inhibit host cell apoptosis.
Mtb inhibits host apoptosis by secreting many effector proteins (Table 1).Mtb tyrosine phosphatase PtpA 47 and DUF732 domain-containing protein kinase (Rv3354) 48 manipulate the ubiquitin system to suppress apoptosis.Recent research has identified other effector proteins, such as Rv2387, which deactivates caspase8/3 in RAW264.7 cells and inhibits macrophage apoptosis, which promotes the survival and growth of M. tuberculosis. 49,63A cell wall-related protein Rv3435c 50 and PE_PGRS18 both enhance the intracellular survival of pathogens by inhibiting cytokine profiling and attenuating cell apoptosis. 51,52MoxR1 inhibits apoptosis by suppressing the expression of MAPK JNK1/2 and cFOS, meanwhile, it also activates the PI3K-AKT-mTOR signaling cascade and autophagy. 53Serine-threonine protein kinase E (pknE) promotes Mtb survival in THP-1 cells by suppressing apoptosis during nitrate stress. 54ne of the ways Mtb manipulates host metabolism reprogramming is through a secreted protein called Rv1813c, which targets mitochondria and inhibits the release of cytochrome c, thus preventing apoptosis and benefiting the survival of the bacterium. 55Another protein, chaperone protein Cpn60.2 (GroEL2), also targets mitochondria to inhibit apoptosis. 56Additionally, Mtb inhibits apoptosis through regulating pathogen genomics, thereby increasing the virulence and survival of Mtb. 57,58,64verall, the role of apoptosis in tuberculosis infection is complex, and the contradictory findings in the literature regarding the induction or inhibition of apoptosis by Mtb may be attributed to factors such as the function of mitochondria, and the type of Mtb effectors during the study.

| Therapeutic strategies targeting host apoptosis
Various measures targeting the activation or inhibition of apoptosis have been explored as potential strategies for TB therapy.Including the application of small molecule compounds, protein peptides, and nanoparticles.
For example, Ac-93253, a small molecule compound, targets the regulation of mitochondrial Bcl2 and Bax expression and promotes caspase3 activation, leading to DNA fragmentation and apoptosis, thereby inhibiting pathogen growth and survival. 65ZnO-se nanoparticles exert their action in various ways including disrupting ATP production, increasing reactive oxygen species (ROS), disrupting mitochondrial membrane potential, and inhibiting PI3K/Akt/mTOR signaling activation, ultimately promoting apoptosis and autophagy in host cells. 66APTX4870 peptide inhibits Mtb secretion of Mycobacterium-derived mycolic acid in THP-1 cells, attenuating inflammatory responses by inhibiting apoptosis and inducing autophagy. 67 A B L E 1 List of Mtb effectors or factors regulating host cell apoptosis.

Outcomes
Ref.

Rv3090
Mouse peritoneal macrophages Induces TNF-a,IL- It is intriguing to note that these interventions involve two distinct modes of death: apoptosis and autophagy.This suggests the possibility of a mechanism that facilitates the interconversion between these modes of death.

| Autophagy
Autophagy is a lysosome-dependent PCD and has proved to be an innate immune response to the removal of intracellular pathogens. 68,69The complete process of autophagy involves the initiation of autophagy through UNC-51-like kinase 1 (ULK1) phosphorylation of the ptdlns3K nucleation complex, the ATG12 and light chain 3 (LC3) conjugation system for autophagosome formation, ATG14-mediated autophagosome-lysosome fusion, and degradation and recycling of macromolecular components by acidic hydrolases for cellular utilization. 70utophagy plays a crucial role in host defense by eliminating intracellular pathogens.In this process, autophagosomes engulf bacteria and fuse with lysosomes to form autolysosomes, leading to the degradation of bacteria. 71ecent studies have shown that autophagy is significant in various diseases including cancer, metabolic diseases, neurological disorders, autoimmune diseases, and infectious diseases. 72In this review, we focus on the role of autophagy in Mtb-host interactions.

| Host-induced autophagy in response to Mtb infection
Autophagy plays a crucial role in targeting and eliminating pathogenic bacteria.The initiation of autophagy in response to Mtb infection requires the participation of various autophagy-related proteins.Some of these proteins involved in inducing autophagy include ATG16L1, ATG5, ATG7, and ATG14. 73Additionally, host receptor proteins like p62 (SQSTM1), calcium binding and coiled-coil domain 2 (CALCOCO2/NDP52), Beclin-1, and microtubule-associated protein 1A/1B-light chain 3 (LC3) are involved. 8ATG5, for instance, functions as part of the E3 ligase responsible for the lipidation of ATG8 proteins.Studies using mouse models have demonstrated the protective role of ATG5 in host defense against infection. 74,75oreover, ATG7 and ATG14 have also been found to actively participate in phagosome-lysosome fusion. 76ecently, receptor-interacting protein kinase-3 (RIP3), a crucial kinase in necroptotic cell death signaling, has been found to be involved in autophagy.RIP3 enhances the binding of p62 to ubiquitylated proteins and LC3, which promotes autophagy in BCG infection. 77uring the early stage of Mtb infection, tumor necrosis factor-like weak inducer of apoptosis (TWEAK) binds to its receptor fibroblast growth factor-inducible 14 (Fn14), promoting Ca 2þ endocytosis and activating autophagy through the AMPK signaling pathway cascade.Another study on Ca 2þ influx suggests that the secreted protein Wnt5a mediates Wnt/Ca 2þ signaling to promote autophagy. 78Additionally, tyrosine kinase-like orphan receptor 2, involved in lipid metabolism, regulates cholesterol metabolism and contributes to the induction of autophagy. 79

| Mtb effectors induce host autophagy
The direct contact of Mtb with the host cytosol induces autophagy and promotes pathogen clearance. 80The ESX-1 secretion system of Mtb promotes phagosome permeability, thereby activating Ub-mediated STING-dependent xenophagy and enhancing anti-infection immunity. 81Additionally, the recognition of Mtb DNA by the host's cytosolic DNA sensor, cyclic GMP-AMP synthase, promotes the production of type I IFN. 81utophagy serves as the primary mechanism for the host to clear pathogens.However, it remains unclear whether Mtb can benefit itself by inducing autophagy.Interestingly, a recent study discovered a dual-functional bacterial effector called eukaryotic-type protein kinase G (PknG), which is one of the eleven eukaryotic-type serine-threonine protein kinases (STPKs) in Mtb.PknG induces autophagy by inhibiting AKT while also targeting the host's small GTPase RAB14 to impede autophagosome maturation.This induced autophagy and targeted intervention of autophagosome maturation ultimately promote the survival of intracellular bacteria. 82This study suggests that the activation of autophagy is only the beginning of anti-infection immunity, while the formation of mature autophagosomes and their fusion with lysosomes are key to killing pathogenic bacteria.

| Mtb effectors inhibit host autophagy
Mtb has developed various mechanisms to evade host immunity by inhibiting autophagy.Previous reviews have described Mtb factors such as ESAT-6, protein tyrosine phosphatase A (PtpA), multiple PE/PPE protein family members, and probable ligase (CpsA) that contribute to resistance to host autophagy. 83s research has advanced, additional factors promoting immune escape have been discovered.For instance, the Mtb Beijing strain (BJN), known for its resistance to autophagy, up-regulates the components of the BORC complex (Kxd1, Plekhm2, and Kinesin-1) to move lysosomes toward the cell periphery. 84Moreover, the upregulation of KatG inhibits autophagic clearance by blocking autophagy-lysosome fusion and inhibiting ROS release. 85KatG is also an NIH resistance gene.Additionally, a conserved hypothetical protein Rv0790c 86 and Mycobacterial acyl carrier protein (AcpM; Rv2244) 87 both promote bacterial survival by activating the mTOR pathway and downstream targets.Mtb can inhibit autophagy as a means of escaping host defenses by modulating host factors.Mtb regulates host SIRPα, which inhibits autophagy and induces necrosis in macrophages.Additionally, various microRNAs, such as miR-25, 88 miR-431-3p, and miR-1303, 89 are involved in this process.
Overall, Mtb inhibits host autophagy through multiple mechanisms, including the activation of the mTOR signaling pathway, inhibition of TLR signaling, and prevention of autophagosomal lysosomal fusion.However, there is still a lack of comprehensive understanding regarding the specific components of Mtb that enable it to evade autophagy.Further studies should focus on characterizing these Mtb effectors involved in the regulation of autophagy.

| Therapeutic strategies targeting host autophagy
New therapies to manipulate host autophagy using a variety of autophagy-inducing compounds have shown a recent trend. 90Various drugs, including isoniazid and pyrazinamide (first-line tuberculosis drugs) and other compounds and plants mentioned in previous studies, have been reported to target and activate host autophagy. 91,92ecent studies have shown a variety of old remedies to be remarkably effective in warding off infections, including clonamine, 91 the antidepressant drug amoxapine, 93 and so on.Berbamine (BBM), an FDA-approved drug, inhibits drug-sensitive and resistant Mtb growth by promoting macrophage autophagy.It is considered to be a calcium-channel blocker and holds promise as a new anti-tuberculosis drug. 94A specific, non-toxic, smallmolecule inhibitor called SMIP-30 acts on metaldependent protein phosphatases (PPMs) and promotes the phosphorylation of p62-SQSTM1.This leads to the facilitation of host autophagy, resulting in an antibacterial effect. 95Additionally, natural components found in plants can target autophagy-related genes (ARGs) in the host, thereby enhancing autophagy.For example, sulforaphane (SFN), a naturally occurring substance found in broccoli sprouts, has been shown to reduce the intracellular bacterial load by enhancing autophagy in THP-1 cells.Moreover, SFN could significantly induce the expression of forkhead box protein O1 (FOXO1), which is an autophagy-related gene and a potential gene target for TB immunotherapy. 96umerous studies have extensively investigated mTOR signaling, ERK1/2 signaling, autophagy protein assembly, and ROS release as potential therapeutic targets.In Table 2, we provide a summary of the identified interventions for future reference in subsequent studies.

| PYROPTOSIS
Pyroptosis is a pro-inflammation PCD that depends on the activation of caspases and cleavage of gasdermins (GSDMs), resulting in the perforation of the cell membrane.This process is characterized by cell swelling, a significant release of cellular contents such as LDH and inflammatory factors, and further activation of the inflammatory response. 102Here are two main mechanisms involved in the initiation of pyroptosis.One is the classical pyroptosis pathway mediated by caspase-1 cleaved GSDMD, the other is the non-classical pathway which depends on other caspases and GSDMs. 103he impact of pyroptosis on infection outcomes has been a subject of controversy.It has been observed to facilitate sterilization during the early stages of infection, but an excessive inflammatory response can also lead to sustained tissue damage, which in turn promotes bacterial spread. 104Therefore, further research is needed to understand the mechanisms of host cell pyroptosis and the effectors of Mtb that regulate infection outcomes.

| Host-induced pyroptosis in response to Mtb infection
The activation of inflammasome plays a crucial role in Mtb-induced host pyroptosis. 105Pathogens or self-ligands activate nucleotide-binding oligomerization domain-like receptors (NLRs) or absent in myeloma 2 like receptors (ALRs), these receptors then assemble with the adapter apoptosis-associated speck-like protein containing a CARD (ASC) to form inflammatory vesicle complexes.These complexes further activate caspase1, which leads to the cleavage of pro-inflammatory cytokines interleukin (IL)-1β and IL-18 as well as the GSDMD, triggering pyroptosis. 106he NOD-like receptor family pyrin domain containing 3 (NLRP3) is the most elaborate inflammasome during Mtb infection, which is relatively conserved in different types of macrophages. 107,108In addition to the NLRP3 inflammasome, activation of the absent in melanoma 2 (AIM2) has been observed in various Mycobacterium species. 109,110AIM2 recognizes pathogen DNA, leading to the cascade activation of caspase1 and secretion of mature IL-1β and IL-18. 111Interestingly, AIM2 is not activated when BMDCs and BMDMs are infected with Mtb. 109Furthermore, the main focal death protein currently associated with Mtb infection is GSDMD. 104However, further investigation is needed to determine if other GSDMs are involved in Mtb-host interactions and how they modulate host immunity.

| Mtb effectors induce host pyroptosis
The effector molecules that promote pyroptosis are mainly several virulence proteins (Table 3).During the phagocytosis of bacteria, a contact-induced plasma membrane damage response occurs mediated by the type VII secretion system (ESX-1). 106ESAT-6 acts as a substrate for the ESX-1 system and can induce pyroptosis by activating caspase-1. 107The RD3-encoded secreted protein Rv1579c (EST12) and its critical amino acid Y80 directly interacts with the receptor for activated C kinase 1 (RACK1) and trigger macrophage pyroptosis.Moreover, an EST12deficient strain (H37RvΔEST12) displayed higher susceptibility to Mtb infection in vitro and vivo. 104This highlights the role played by Mtb-EST12 in stimulating the host immune response, which enhances mycobacterial clearance.
Additionally, PE/PPE proteins are also involved in activating inflammasome.For example, PPE13 activates the NLRP3 inflammasome, thereby inducing caspase-1 cleavage and IL-1β secretion in J774a.1,BMDMs, and THP-1 macrophages. 112Recent studies have shown for the first time that PE_PGRS19 induces pyroptosis through a non-classical Caspase-11/GSDMD pathway in macrophages and promotes the survival of the bacterium. 113

| Mtb effectors inhibit host pyroptosis
How pathogens evade the host immune response by targeting pyroptosis has not been extensively explored (Table 3).Several proteins of Mtb have been described as associated with inhibiting host pyroptosis, including serine hydrolase Hip1, 114 the serine/threonine kinase PknF, 115 and the membrane protein Rv3364c. 116Recent studies have revealed that the eukaryotic-like effector protein PtpB (Rv0153c) from Mtb, which targets and inhibits the membrane localization of the N-terminal cleavage fragment of GSDMD (GSDMD), thus preventing GSDMD-dependent pyroptosis. 117This study uncovers a novel pathogen evasion mechanism, specifically, pathogens utilize Ub to hinder host pyroptosis by modifying the host membrane's phospholipid composition.Additionally, Mtb can inhibit pyroptosis by regulating certain genes and promoting the dissemination of the bacterium.One of these genes is Rv0198c (zmp1), which encodes a putative Zn 2þ metalloprotease.Zmp1 inhibits caspase1 activation, suppresses pyroptosis and promotes the survival and virulence of Mtb. 118Meanwhile, Lnc-EST12 has been shown to decrease the expression of proinflammatory cytokines and inhibit the NLRP3 inflammasome and GSDMD pyroptosis-IL-1β immune pathways. 119Another study found that the gain-of-function allele Lrrk2G2019S disrupts mitochondrial homeostasis and reprogramms cell death pathways in macrophages.In Lrrk2G2019S macrophages, mtROS promotes pyroptosis shift toward RIPK1/RIPK3/MLKL-dependent necroptosis, thereby causing severe inflammatory and pathological consequences. 120This study suggests that the switch between pyroptosis and necrosis may be associated with mitochondrial homeostasis.

| Therapeutic strategies targeting host pyroptosis
Two plant herbal medicines can target host pyroptosis to modulate infection outcomes.One of them is tanshinone IIA (Tan), which can decrease inflammation by inhibiting the activation of the NLRP3.Hence, Tan can be used as an adjuvant drug for tuberculosis treatment by modulating the host immune response. 121Another one is Baicalein (5,6,7-trihydroxyflavone), which enhances host resistance by activating autophagy and blocking the assembly of AIM2, NLRP3 inflammasome, thereby inhibiting pyroptosis. 122urthermore, ursolic acid, a natural pentacyclic triterpenoid carboxylic acid, synergistically inhibits the Akt/ mTOR and TNF-α/TNFR1 signaling pathways, promotes autophagy and inhibits pyroptosis and necroptosis of macrophages. 123This study suggests that multiple death modes are involved in Mtb infection, but the molecules linking these three death modes and their regulation are still unclear.

| FERROPTOSIS
The concept of ferroptosis was first proposed and named in 2012. 124Ferroptosis is defined as an iron-dependent lipid peroxidation process, which is categorized as a form of PCD.In recent years, there has been a growing interest in studying the role and mechanism of ferroptosis in various diseases. 125One of the prominent characteristics of ferroptosis is lipid peroxidation, which involves a series of free radical chain reactions. 126Iron ions and ROS derived from lipid and iron metabolism play crucial roles in driving ferroptosis.
Ferroptosis is dependent on the Glutathione peroxidase 4 (GPX4)) pathway and the non-GPX4 pathways.Glutathione peroxidase 4 functions by reducing the level of intracellular lipid peroxide through the process of lipid detoxification. 127Glutathione (GSH) serves as a cofactor for GPX4.Inhibiting GSH can hinder the action of GPX4, 127 furthermore, a GSH-independent mechanism of regulating GPX4 has been identified, and inhibiting mammalian rapamycin complex 1 (mTORC1) activity reduces intracellular levels of GPX4. 128Additionally, System Xc-, FSP1/coenzyme Q10, dihydroorotate dehydrogenase (DHODH), and GCH1/BH4 all can induce ferroptosis, which are all pathways independent of GPX4. 125,129er the past few years, numerous studies have shown that ferroptosis is closely associated with the development and progression of a wide range of diseases. 126,130,131However, the role of ferroptosis in infectious diseases, particularly in TB, remains uncertain.

| Mechanisms of host ferroptosis during Mtb infection
As early as 1872, it was suggested that elevated iron levels were linked to a higher risk of active tuberculosis in patients.Subsequent reports confirmed a strong correlation between iron levels and tuberculosis. 132,133During Mtb infection, ferroptosis is triggered by various factors, such as oxidative stress, GPX4 inactivation, depletion, and lipid peroxidation associated with iron accumulation. 134 recent study demonstrated that the use of ferrostatin-1, a ferroptosis inhibitor, led to a lower bacterial load in infected animals.These findings highlight the role of ferroptosis in host-pathogen interactions and suggest that targeting the Gpx4/GSH axis could be a promising approach for host-directed therapy. 132eme oxygenase-1 (HO-1), typically regarded as a cytoprotective molecule in infectious diseases, has recently been suggested to play a detrimental role.During Mtb infection, the accumulation of ROS and lipid peroxides can promote tissue damage and the spread of pathogenic bacteria.However, HO-1 can increase the content of ferrous metals, thereby promoting the survival of Mtb. 134,135

| Mtb effectors induce host ferroptosis
As research progresses, the factors of Mtb that affect host ferroptosis are gradually being resolved, including several proteins (Table 4).Protein tyrosine phosphatase A has been identified as an effector protein for ferroptosis, which inhibits the expression of genes related to GSH metabolism.Additionally, in the nucleus, PtpA interacts with the protein arginine methyltransferase 6 (PRMT 6), which is responsible for inducing ferroptosis.This interaction leads to the repression of GPX4 expression by inducing the formation of histone H3 arginine 2 (H3R2me2a), resulting in the onset of ferroptosis and the spread of pathogenic bacteria. 136Peptidyl prolyl isomerase A (PpiA) is a marker for latent tuberculosis. 139Recent research has revealed that PpiA can interact with host integrins and upregulate the expression of matrix metalloproteinases, enhancing Mtb survival and promoting granuloma destruction in parallel. 137 of 20 Another secreted protein of Mtb, the secretory protein Rv1324, has been associated with ferroptosis.Rv1324 is possibly against the reactivity of ROS and RNS.It increases the activation of ferroptosis, enhancing the survival of Mycobacterium smegmatis within host cells and inducing inflammation and pathological damage in the mouse lung. 138This study deepens our understanding of Mtb survival and pathogenesis, offering new targets for anti-TB treatment.

| Therapeutic strategies targeting host ferroptosis
A variety of agonists and inhibitors of ferroptosis have been applied to the treatment of tumors, neurological disorders, organ damage, and other diseases.In clinical studies, patients with TB have shown some therapeutic benefits from ferroptosis-related drugs such as vitamin E or selenium enzymes. 140,141A recent study has identified cytokine signaling 1 (SOCS1) as a ferroptosis-related differentially expressed gene that may play a role in TB therapeutic efficacy and drug resistance.Additionally, SOCS1 has been associated with ferroptosis and bacterial survival. 142he role of Mtb effectors involved in host ferroptosis and its molecular mechanisms are currently in the preliminary exploratory stage.Further investigation into the host and Mtb factors associated with ferroptosis will enhance our understanding of the role of ferroptosis in Mtb infections.Moreover, this research may lead to the identification of new diagnostic and therapeutic targets for TB treatment.

| NECROSIS
Necrosis was first identified as a form of passive cell death resulting from intrinsic or extrinsic factors, including physical and chemical stimuli.The characteristic morphology of necrosis exhibits uncontrolled cell death, characterized by the loss of cell membrane integrity, cell swelling, cell disintegration, release of cell contents, and induction of inflammation. 143Activated TNFR and Fas and Toll-like receptors (TLR3, TLR4) 144 bind to FADD, TRADD, and TRIF ligands and interact with RIPK1, cas-pase8, or caspase10, which then activate mixed-lineage kinase domain-like protein (MLKL) to induce cell necrosis. 143Additionally, the cytoplasmic nucleic acid sensors RIG-I and cGAS/STING, as well as TLR4, contribute to necrosis. 143These regulators of necrosis play a crucial role in the regulation of various diseases.
It is widely recognized that the progression of TB is promoted by Mtb-induced host necrosis. 145However, there is a lack of research that explores host or bacterial factors that can inhibit host necrosis.Furthermore, inducing necrosis does not only benefit bacterial immune escape.Therefore, more research is needed to clarify the impact of necrosis in host-pathogen interactions to provide a basis for controlling TB progression.

| Host-induced necrosis in response to Mtb infection
Several studies have demonstrated that Mtb-induced neutrophil and macrophage necrosis contribute to the growth and persistent infection of Mtb. 146During Mtb infection, various effector proteins such as RIPK1/RIPK3, MLKL, TNFR1, and ZBP1 are significantly upregulated in response to necrotic signaling. 147Interestingly, it has been observed that the intervention of MLKL or RIPK1 in macrophages does not completely reverse necrosis. 147nterestingly, it has been observed that the intervention of MLKL or RIPK1 in macrophages does not completely reverse necrosis.
A recent study has shown that cyclophilin D, a mitochondrial matrix protein, interacts with TNF-RIPK1-RIPK3 and increases mitochondrial ROS production, which is necessary for necrosis along with cyclophilin D. 148 Oxidative stress activates cyclophilin D and causes sustained opening of the mitochondrial permeability transition pore complex, resulting in the subsequent impact on mitochondrial membrane potential and function. 149Further studies have demonstrated that mitochondrial ROS and cyclophilin D are essential for TNF-αinduced mitochondrion-intrinsic necrosis. 150

| Mtb effectors induce host necrosis
In contrast to apoptosis, Mtb-induced necrosis of host cells may be more favorable for the spread and growth of pathogenic bacteria. 151Moreover, the replicate speed of Mtb in necrosis cells is faster than in living cells. 152everal virulence proteins are the main effectors that induce host necrosis in Mtb, including secreted proteins, toxins, and PE/PPE proteins (Table 5).
The ESX-1 secretion system and its substrate ESAT-6 are required for Mtb-induced necrosis. 153,154Pathogens that fail to secrete ESAT-6 and CFP-10 are unable to induce macrophage damage. 161However, it is worth considering that Mtb can secrete ESAT-6 and CFP-10 early in macrophage phagocytosis, but macrophage necrosis and Mtb dissemination occur over several days. 162Channel protein with a necrosis-inducing toxin (CpnT/Rv3903c), which possesses NAD þ glycohydrolase activity and induces necrosis. 155This suggests that Mtb can activate different necrotic pathways by switching the TNF/TNFR1/RIPK1 cascade to avoid the immune response.Meanwhile, some members of the PE/PPE protein family are involved in the induction of host necrosis. 156For example, PPE31 is an important regulator of host immunity, localizing to the cell membrane and modulating host JNK signaling to promote the survival of Mtb. 157Rv2626c interacts with PPE68, and promotes necrosis of host cells early in the course of infection. 158The PE25/PPE41 protein complexes, secreted by a similar/unique transport system (Type VII), regulate virulence and disease reactivation. 159he molecular mechanisms underlying host necrosis during exacerbation of TB by Mtb and its effectors are not yet fully understood.Further investigation into the functions of Mtb proteins and lipids in inducing host cell necrosis could shed light on the mechanisms of its virulence.

| Therapeutic strategies targeting host necrosis
There have been only a limited number of studies that have focused on targeting host necrosis as a treatment for TB.Streptomycin, a first-line anti-tuberculosis drug, has been shown to reduce cell necrosis in human lung epithelial cells (A549). 163A recent study demonstrated that manganese can increase the accumulation of TNF-a in macrophages, thereby inducing necrosis and inhibiting Mtb survival.This study highlights the significant role of Mn 2þ in host cell defense against Mtb infection, Mn 2þ may be a drug cofactor in tuberculosis treatment and the development of new-generation drugs and vaccine adjuvants. 164Meanwhile, it revealed that the outcome of necrosis not only benefits Mtb but also inhibits Mtb survival.

CELL DEATH MODES AND THE POTENTIAL MODULATORY TARGET
The discovery of multiple modes of cell death has significantly enhanced our understanding of the development of various diseases.Numerous studies on hostpathogen interactions have demonstrated a diverse range of transitions and interactions between different modes of cell death. 8he same Mtb factor can induce different cell death modes and lead to different immune outcomes.For instance, the Mtb-secreted protein ESAT-6 has been found to induce apoptosis, 36 pyroptosis, 107 autophagy, 98 and necrosis. 153Meanwhile, Mtb tyrosine phosphatase PtpA can promote its survival in THP-1 cells by inhibiting apoptosis, 47 inhibiting autophagy, 99 and promoting ferroptosis 136 to promote self-survival and propagation.The switch between different types of cell death can either favor Mtb or be used to kill Mtb, and various interventions have been explored to target and inhibit these cell deaths to combat Mtb.For example, APTX4870 peptide, 67 BAG2, 165 and NF-E2 166 have been shown to inhibit apoptosis and promote autophagy, thereby benefiting host immunity.The occurrence and switching of multiple death modes largely determines the outcome of host immunity, but the molecular switches between different crosstalk and regulation of different death modes are unclear.
Mitochondria are the energy metabolism center of the cell and are involved in regulating various forms of Mtbinduced cell death.They serve as the initial triggers for endogenous apoptosis, controlling processes such as cytochrome release, BAX/Bcl2 activation, ROS production, and caspase activation. 167Rv0335c, a protein of Mtb effector, could induce host apoptosis by inducing mitochondrial depolarization, increasing the level of cytoplasmic Cyt C, and reducing the concentration of intracellular ATP. 168A recent study has revealed that Mycobacterium bovis induces mitophagy to suppress host autophagy for its intracellular survival. 169Mitochondrial damage-associated molecular patterns and mitochondrial pumps can trigger the release of IL-1β and initiate pyroptosis through the activation of the inflammasome. 170he generation of ROS by mitochondria plays a role in initiating necroptosis by facilitating the autophosphorylation of RIPK1.Furthermore, mitochondrial hyperpolarization and elevated ROS levels have been observed to enhance lipid peroxidation and induce ferroptosis. 171espite the growing interest in mitochondria as key regulators of antimicrobial defense and cell death, our understanding of how mitochondrial perturbations drive protective or pathogenic immune responses in human disease remains limited. 172,173It is interesting that targeting the regulation of mitochondria can have complex effects on cell death outcomes.For instance, Mtb effectors can promote the survival of Mtb by targeting mitochondria to induce apoptosis, 41,[43][44][45][46] however, they can also promote the survival and dissemination of Mtb by targeting mitochondria to inhibit apoptosis. 57Moreover, the gainof-function allele Lrrk2G2019S (leucine-rich repeat kinase 2) promotes mitochondrial ROS release, which can bind to GSDMD and cause perforation of the mitochondrial membrane, contributing to necroptosis. 120The gainof-function allele Lrrk2G2019S alters cell death pathways by disrupting mitochondrial homeostasis in macrophages.These findings suggest that mitochondria could be a potential target for shifting different host cell death modes and regulating cell death outcomes.
While there is currently no direct evidence supporting the significant role of mitochondria in driving different modes of death, their widespread involvement in multiple deaths presents intriguing and unexplored possibilities.By elucidating the association between mitochondria and various cell death modes, we can enhance our comprehension of the mechanisms of Mtb to evade host immunity and facilitate the exploration of new clinical therapeutic approaches.

| CONCLUSION AND OUTLOOK
In recent years, there has been significant progress in understanding the interactions between Mtb and the host innate immune system.The discovery of various cell deaths has greatly enhanced our knowledge of the disease's pathogenesis.In this review, we have discussed the role of multiple cell death modes in host-pathogen interactions, mainly involving several of the more studied types of PCD and necrosis, other types of cell death, such as ETosis, are not addressed in this paper.As shown in Figure 1, different cell death modes are regulated by various Mtb virulence effectors and influence immune outcomes.Interestingly, one virulence effector can be critical to more than one cell death mode.In addition to discussing the molecular mechanisms of Mtb effectors that regulate host cell death and intervene in host immune outcomes (Figure 1), we highlight the potential value of multiple strategies targeting host cell death pathways in TB therapy (Table 6).This review summarizes relevant study details in Tables 1-6 to inform readers of further research.
Going forward, given the diversity of cell death modes induced by Mtb and the complexity of different cell death modes crosstalking with each other, an important area of research will be to identify the key targets that precisely regulate the onset of different cell death modalities and outcomes.Meanwhile, the ability of Mtb to adapt to malignant environments and actively manipulate host cell death signaling pathways plays a crucial role in its survival and persistence within immune cells.Therefore, further studies on the mechanism of Mtb-induced host death are necessary, especially the role of ferroptosis during Mtb infection.Apoptosis, pyroptosis, autophagy, necrosis, and ferroptosis are all involved in host-Mtb interactions.Apoptosis and autophagy generally enhance host immunity, while pyroptosis, necrosis, and ferroptosis promote the survival and dissemination of Mtb.However, recent research suggests that outcomes resulting from different cell death modes are not consistent and that there is a surprising flexibility in the various pathways of cell death.Although we have extensively explored the role of multiple cell deaths in host-pathogen interactions, there are still limitations to our understanding.The spatiotemporal characterization of the various modes of cell death induced by Mtb, the intrinsic link between different cell death modes, and the key targets that track cell death modes and outcomes are still not well understood, necessitating further study.Although we have speculated on the potential of mitochondria as a target for manipulating different cell death, this is based on existing studies and lacks direct evidence.Research has shown that various pathogens can evade host immunity by affecting mitochondrial dynamics and function, 187 and therefore, it is necessary to conduct studies on the role of mitochondrial morphology and structure, mitochondrial dynamics, and mitochondrial metabolism in regulating the mode of death during Mtb infection.Meanwhile, various therapeutic strategies targeting host cell death pathways are effective at the cellular or animal level, but further research is needed to validate the effectiveness, safety, and feasibility of these therapies.
Finally, our understanding of the mechanism of Mtbinduced host death is just the tip of the iceberg, an indepth understanding of the different mechanisms of cell death, identifying and interfering with target molecules capable of modulating cell death modes, is important for the diagnosis and therapies of TB.

F I G U R E 1
Overview of host cell death pathways regulated by Mtb through various virulence factors.Different cell death modes are induced during Mtb infection, resulting in opposite immune outcomes.On the one hand, Mtb virulence factors-induced cell death decreases Mtb survival, and on the other hand, Mtb virulence factors-regulated cell death can also enhance Mtb survival, replication, or dissemination.Red arrows represent induction, and black inhibitors represent inhibition.Created with biorender.com.Mtb, Mycobacterium tuberculosis.T A B L E 6 List of potential therapeutic strategies targeting different host cell death modes.
List of Mtb effectors or factors regulating host cell autophagy.
List of Mtb effectors or factors regulating host cell pyroptosis.
List of Mtb effectors or factors regulating host cell ferroptosis.
T A B L E 4 List of Mtb effectors or factors regulating host cell necrosis.